The present invention relates to a method for producing a fiber preform by means of winding and a set of tools for implementing the method.
In FIGS. 1 and 2, a frame 10 of an aircraft window produced from composite material has been illustrated. This frame 10 is obtained from a fiber preform comprising a plurality of fiber sub-preforms.
For the remainder of the description, the term “preform” is intended to refer both to a preform and to a sub-preform which is capable of being associated with other sub-preforms in order to form a preform.
Some preforms 12 of the frame 10 have a continuous profile over the entire periphery of the frame, in particular in order to limit the radial deformations which tend to increase the perimeter of the frame.
According to an embodiment illustrated in FIGS. 3 to 5, a preform 12 is obtained from at least one thread 14, in particular of carbon, wound with a degree of tension about a winding axis 16 between two flanges 18.1 and 18.2.
According to this construction technique, the cross-section of the preform 12 in a plane which contains the winding axis 16 is substantially constant over the entire periphery of the preform.
The term “thread” is intended to refer to at least one filament. According to an embodiment, a thread of carbon is a strand of several thousands of filaments of carbon of a few micrometers (for example, in the order of 7 μm).
Advantageously, the filaments are powdered resins.
After the winding phase, the preform 12 is subjected to a thermal cycle in order to confer thereon a degree of cohesion as a result of the powdered resin in order to be able to be manipulated.
Subsequently, the different preforms are assembled and are subjected to a thermal cycle which is intended to achieve the polymerization of the assembly in order to obtain a frame.
According to a significant constraint, each preform must have a thread density which is homogeneous over the entire cross-section and over the entire circumference. Furthermore, each cross-section of the preform must have a profile in accordance with that desired, in particular in the region of the corners.
According to an embodiment which is not illustrated, the cross-section of the preform may be rectangular and delimited by two faces which are planar and perpendicular relative to the winding axis. For this geometry, a homogeneous thread density is obtained over the entire cross-section and over the entire circumference.
According to other variants, the preform 12 may have L-shaped, S-shaped or other cross-sections.
In this instance, the preform 12 comprises two opposing faces 20.1 and 20.2 which are non-planar and spaced apart. In a parallel state, the flanges 18.1 and 18.2 delimit a groove 22 with two faces 24.1 and 24.2 which complement the faces 20.1 and 20.2 of the preform, respectively.
For these geometries, some zones 26 of the groove 22 are difficult to access when the thread 14 is wound given the tension thereof. These zones 26 are illustrated in grey in FIG. 5.
Owing to the presence of these zones 26 which are difficult to access, the density of the threads is not homogeneous over the entire cross-section, which leads in particular to shape defects and non-conformity of the component in terms of geometry and dimensions.
In order to overcome these disadvantages, a first operating method of the prior art involves subdividing the preform into sub-preforms 28, 28′, 28″ which are obtained in accordance with the winding technique described above, each sub-preform which is referred to as being simple having a geometry which is compatible with good filling, that is to say, with opposing faces 20.1 and 20.2 which are planar and perpendicular relative to the winding axis 16. Subsequently, the sub-preforms are assembled in a set of tools during a stamping operation, as illustrated in FIG. 6. This stamping operation involves compressing the sub-preforms 28, 28′, 28″ between two dies 30.1 and 30.2 in order to cause the layers of threads to slide laterally in order to obtain the definitive preform.
Even if this allows some defects to be overcome, this operating method is not satisfactory since it leads to a multiplication of the number of winding operations which tends to increase the cost of the component obtained and the time required for it to be obtained.
According to another operating method of the prior art illustrated in FIG. 7, the winding is carried out between two flanges 18.1 and 18.2 with a spacing between the two faces 24.1 and 24.2 greater than the theoretical value. In this manner, a gap e greater than 0 is provided between the two flanges 18.1 and 18.2. After the winding operation, the two flanges are moved together in order to eliminate the gap e and to obtain a spacing between the two faces 24.1 and 24.2 in accordance with the theoretical value. This movement together is intended to overcome the lack of threads in the zones 26. However, tests have shown that the zones 26 are not adequately filled and that the geometries in these zones 26 are not sufficiently precise (insufficiently distinct angle).